1. Field of the Invention
The present invention relates to the rotor of a compressor or turbine of a gas turbine engine, and is particularly concerned with the fixing of the blades to a disc of the rotor.
2. Summary of the Prior Art
In the compressor stages of small diameter engines, it is known to fasten the blades to the rotor disc by providing the blades with a so-called hammer-head type of root, i.e. a bulbous root which is arranged to fit into a circumferential groove in the rim of the disc having a cross section corresponding to that of the bulbous root.
In more modern engines, the number of blades has been increased, which has necessitated a reduction in the pitch of the blades, and to achieve this it has been necessary to provide the blades with diamond shaped platforms instead of rectangular platforms as previously used.
With these diamond shaped platforms, the centre of aerodynamic thrust on each blade corresponds to a moment which has a tendency to cause the blade to pivot in the groove. This tendency is further accentuated by the presence of circumferential play between the blade platforms to cater for the tolerances in their width and by an increase in the basic clearance resulting from the natural pivoting of the blades.
This pivoting is of course countered by the bulbous roots of the blades, whose active flanks are brought into contact with the sides of the groove in the disc by the action of centrifugal force. However, during the engine starting phase, the centrifugal forces are insufficient to prevent all pivoting of the blades, which results in uneven bearing of the roots in the groove and variations in the setting of the blades.
To avoid this pivoting, present techniques make use of various solutions. For example, one known solution, which is illustrated in FIGS. 1 and 2 of the accompanying drawings, involves providing the inner face of the platform of each blade with rebates 1 and 2 which mate with circumferential ribs 3 and 4 on the rim 5 of the disc, so that the platforms will butt against the disc. This solution has the disadvantage of requiring a thickening of the platform and a widening of the rim of the disc.